The presence of even low levels of spoilage or pathogenic microorganisms in various products can be dangerous and even lethal. Therefore, sensitive, rapid, and simple-to-use methods for detecting microorganisms that can be performed in out-of-the-laboratory settings, and by untrained persons, are preferred over those requiring sophisticated techniques that are laborious, time consuming, or require skilled personnel to carry out. For example, the analysis of genetic information based on the polymerase chain reaction (PCR), a popular method for sensitive detection of bacteria, requires complicated sample preparation protocols performed by skilled personnel. Some alternative methods are known, such as detection of intact bacterial cells using specific antibody-antigen binding, but simple, fast, and sensitive methods are needed.
Since sol particle immunoassays (SPIA) were introduced, target-mediated (i.e., on-target) aggregation of gold nanoparticles (AuNPs) has been used to visually detect the occurrence of some biological events or the presence of small biomolecules such as proteins and DNA. However, using the principle of on-target aggregation of AuNPs for visual detection of large targets such as microorganisms, especially at extremely low concentrations, has proved difficult.